fncache: move fncache writing to be in a transaction Previously the fncache was written at lock.release time. This meant it was not tracked by a transaction, and if an error occurred during the fncache write it would fail to update the fncache, but would not rollback the transaction, resulting in an fncache that was not in sync with the files on disk (which causes verify to fail, and causes streaming clones to not copy all the revlogs). This uses the new transaction backup mechanism to make the fncache transacted. It also moves the fncache from being written at lock.release time, to being written at transaction.close time.